Nowadays, various types of electronic equipment (such as various types of sensors or wireless network nodes) require periodic or continuous wireless charging, because of being implanted in body in advance (including fixed type, such as cardiac pacemaker, and dynamic type, such as capsule endoscopy), or being erected in environment without power supply condition or complicated environment, or not being able to replace batteries because of the requirement of secret installation. However, in wireless charging technology, to ensure a battery to be charged could receive electromagnetic energy effectively, radio wave emitted by wireless charging equipment should have sufficient strength in the area where the battery to be charged is located. But if the wireless charging equipment emits non focused radio wave, a lot of electromagnetic wave energy could not be received by the battery to be charged and would become invalid radiation, not only resulting in the waste of energy, but also causing electromagnetic pollution to human and the surrounding environment. Thus, the goal of wireless charging technology is to realize favorable electromagnetic power focusing in area near the device to be charged by radio waves emitted by charging device.
However, as an electromagnetic emission apparatus of a wireless charging device, spot focused multiple antennas system face the following rigorous challenges when designed.
1. The difficulty of the spot electromagnetic focusing in complex or even unknown and time-varying inhomogeneous propagation environment. Usually, there is highly complex or even unknown and time-varying inhomogeneous environment between the charging equipment and the battery to be charged, leading to the highly unreasonable power distribution of the electromagnetic waves emitted by the charging device, and most of the electromagnetic waves become useless power and lead to electromagnetic pollution. It is highly unrealistic to master the actual inhomogeneous propagation environment in advance and then conduct the electromagnetic focusing design contrapuntally, because the randomness of the propagation medium (such as people's height and body shape), the time-varying characteristics (such as the movement of capsule endoscopes in digestive tract), and the complex multipath propagation effect are difficult to forecast and even harder to accurately grasp. For example, the heterogeneity of lush vegetation has random and time-varying characteristic. As another example, for the wireless charging to the electronic equipment (such as pacemaker and capsule endoscopes) embedded in body, due to the complex nonuniformity within the human body, such as people's height and body shape, breathing habits, ups and downs of the chests when breathing, the movement of the capsule endoscopes in digestive tract, the electromagnetic propagation environment present significant complex nonuniformity and random time-varying characteristic.
2. The battery to be charged is difficult to be found and precisely located, resulting in the difficulty of confirming the focused area in real time. In many applications of wireless charging, the position of the battery to be charged is difficult to be found and precisely located, even changes irregularly. So that the electromagnetic focused area could not be located. Such as the apparatus which is installed secretly, implantable, camouflaged, or installed in the pit, underwater, underground, and the apparatus covered by vegetation and in continually moving and so on.
In contrast, the existing wireless charging technology, including the charging technology based on near field coupling, and the charging technology based on electromagnetic propagation, either does not consider the electromagnetic focusing, or attempts to focus based on the pre-determined simplified propagation model, without reality and practicality; it is even not possible to realize accurate electromagnetic focusing without grasping the accurate position of the battery to be charged and the current non-uniform propagation conditions.
A new wireless energy transfer system, as a typical representation of existing technology, is disclosed in Midfield Wireless Power Transfer for Bioelectronics by Ada S. Y. Poon in IEEE Circuits and Systems. Though this design could change the direction of energy transmission by adjusting the phase of feeding unit, it could not point to the position of electronic devices implanted in body accurately, leading to a decrease in transmission efficiency, and pollution is caused by the electromagnetic power which is not received by receiving antenna, and most of the electromagnetic power would be absorbed by body tissue, elevating the temperature of local tissue, even endangering human safety and health.
Therefore, this system needs further improvement in the application of wireless energy transferring to the device implanted in body.